Current references are used in many electronic circuits to provide a steady known current as a reference to another circuit. For low power applications, low power current references are desirable as reduced power consumption results in longer operation time for battery powered devices. This is particularly important in mobile applications such as mobile computing, mobile telephony, and mobile gaming.
FIG. 1 shows a conventional implementation of a current reference circuit 30. This implementation comprises positive and negative feedback loops for generating the current reference. The positive feedback loop grows (i.e. amplifies) the current in the load resistor RL. The negative feedback loop keeps the load resistor voltage at approximately VTHN, which is the threshold voltage of an N-type transistor.
In FIG. 1, transistors M2, M4 and M5 and the load resistor comprise the positive feedback loop. For example, if the load resistor current rises, transistor M5 mirrors the current increase to transistor M4, causing an increase in the gate voltage of transistor M2 (shown as signal nn). This raises the load resistor voltage and causes a further increase in current. The positive feedback loop grows the load resistor current until restrained by the negative feedback loop. As such, the positive loop gain must be greater than one, or the loop current will never grow to the reference current. In addition, if the negative feedback loop has less gain than the positive feedback loop, the load resistor current will grow without bound (until the load resistor approaches the supply voltage).
In FIG. 1, the negative feedback loop comprises the load resistor, transistor M1, and transistor M2. For example, if the current through the load resistor rises, so does the gate-source voltage of transistor M1 (shown as the signal nbias). This causes transistor M1 to draw more current, pulling down the gate voltage of transistor M2 (signal nn) and, in turn, reducing the voltage and current across the load resistor.
At the circuit's primary operating point, the load resistor operates with the gate source voltage of transistor M1 across it. If transistor M1 is sufficiently large, the current through the load resistor will be approximately VTHN (the threshold voltage of the N type transistor) divided by R (the load resistor), i.e. current equals (VTHN/R).
As this is a self-generating current reference, this circuit also has a second stable operating point with no current flowing. A conventional startup circuit is required to ensure that the current flows in the circuit to put the circuit of FIG. 1 in the operating point where current is flowing.
In the circuit 30 of FIG. 1, the minimum supply voltage at which this circuit will operate can be characterized by:VPWR(minimum supply voltage)=2*VTH+3*VDSAT 
As recognized by the present inventors, a disadvantage of conventional circuits such as the circuit 30 of FIG. 1 is the high supply voltage required. In FIG. 1, the minimum supply voltage may be defined by one of two paths in the circuit. No matter which path requires the larger minimum VPWR, shown in Equations 1 and 2, both paths require greater than two MOSFET voltage thresholds. This is a significant voltage level since, as CMOS processes shrink, the maximum supply voltages are shrinking faster than the MOSFET thresholds such that some of the most recent CMOS processes have supply voltages that are little more than two thresholds. Since gate overdrive or saturation voltages (VDSAT) of 100 mV or more are required to keep MOSFET transistors well in saturation, there is often less supply voltage range available than the conventional circuit requires to function.VPWR−MIN(1)=2VTHN−1,2+3VDSAT−1,2,4  (1)VPWR−MIN(2)=VTHN−1+VTHP−5+3VDSAT−1,2,5  (2)
Accordingly, as recognized by the present inventors, what is needed is a circuit and method for providing a current reference capable of operating with a reduced supply voltage. It is against this background that embodiments of the present invention were developed.